An interleaved parallel connected power converter can improve efficiency and power density while cancel current ripple and improve input or output characteristics. Therefore, the interleaved parallel connected power converter is widely applied in power factor correction circuits, inverter circuits and direct-current (DC) converting circuits. A typical two-phase interleaved circuit is shown in FIG. 1, where each phase of the circuit includes a separate filter inductor, denoted as L1 and L2 respectively. A dedicated control loop needs to be designed in the interleaved parallel connected converter to achieve current sharing between the inductors for the respective phases.
A multi-state switching circuit is a new type of circuit developed based on the interleaved parallel connection structure. A typical three-state switching circuit is shown in FIG. 2. The circuit has a structure similar to the interleaved parallel connection structure, except for two phases sharing one coupling inductor, and a separate inductor, i.e. inductor L in FIG. 2, is further arranged between the coupling inductor and the input (or the output). Current sharing between the interleaved parallel connected circuits can be achieved by providing the coupling inductor.
In the three-state switching circuit shown in FIG. 2, different combinations of ON states of four switching transistors form three equivalent operation states, including a state in which an upper transistor and a lower transistor are ON simultaneously, a state in which two upper transistors are ON simultaneously, and a state in which two lower upper transistors are ON simultaneously, based on which the three-state switch is named. As compared with the interleaved parallel connection structure, current sharing can be achieved automatically between the circuits for respective phases without current sharing control due to the presence of an auto-transformer, thus a current sampling circuit is not needed. In addition, the filtering inductor L and the coupling inductor may be optimized independently based on working conditions thereof, in order to improve converting efficiency. Besides, according to power requirement, the three-state circuit may be extended to a multi-state circuit such as a typical four-state switching circuit shown in FIG. 3.
In order to further improve power density and overall efficiency, the inductor L in FIG. 3 may be replaced with a leakage inductance of the coupling inductor Lcoupling. In this way, an inductor element can be eliminated, and the new component is referred to as “coupling inductor with integrated inductor L”. FIG. 4 shows a cross-sectional view of a structure of a typical four-state coupling inductor with integrated inductor L. The structure includes a magnetic core including three magnetic columns for three phases, and three windings of the coupling inductor, i.e., a winding A, winding B and winding C, which are respectively wound on the three magnetic columns.
Current waveforms in respective windings of the coupling inductor in the multi-state switching circuit include a power frequency current and a high frequency ripple current superimposed thereon. And magnitudes and phases of the power frequency currents in respective windings are the same. Therefore, magnitudes and phases of power frequency magnetic fluxes generated by respective inductors in the respective magnetic columns are the same. As shown by the magnetic lines represented by dashed lines between the magnetic columns in FIG. 4, the magnetic lines generated by each of the three inductors counteract the magnetic lines generated by other inductors after flowing through the other two magnetic columns. In theory the power frequency magnetic fluxes of the three windings counteract one another completely. However, since magnetic paths for the three magnetic cores have different lengths, the magnetic fluxes generated by the power frequency currents in the three windings cannot counteract one another completely. The power frequency magnetic flux is similar to a DC bias with respect to high frequency current, and may cause saturation of the magnetic core. In order to prevent the saturation, a large air gap is needed in the magnetic core. In this case, an inductor loss is increased, which is disadvantageous for efficiency optimization.